1. Field of the Invention
This invention relates to a method of producing a grain oriented silicon steel sheet having excellent magnetic properties, and more particularly to an improvement of magnetic flux density among the magnetic properties in the grain oriented silicon steel sheet.
2. Related Art Statement
In the grain oriented silicon steel sheet mainly used as a core material for transformers and the like, it is required that the magnetic flux density obtained at a predetermined magnetization force be high and also the iron loss obtained at a predetermined magnetic flux density be low. In this connection, the magnetic flux density B.sub.8 (T: tesla) at the magnetization force of 800 A/m and the iron loss W.sub.17/50 (W/kg) at the magnetic flux density of 1.70 T and the frequency of 50 Hz are generally adopted.
In order to improve the magnetic properties inclusive of the above two properties, many studies have been made up to the present. Particularly, good results are obtained to a certain extent by the adjustment of chemical composition in the starting material, improvements of hot rolling process, cold rolling process and heat treatment, and the like.
Heretofore, good magnetic properties of the grain oriented silicon steel sheet have been obtained by hot rolling a starting material of a low carbon steel containing usually 2.5.about.4.5 wt % (hereinafter merely shown by %) of Si and a slight amount of an inhibitor forming element such as Mn, S, Se, Sb, Al, Sn, N, B or the like, subjecting the hot rolled sheet to a heavy cold rolling step or to a combination of two cold rolling steps and an intermediate annealing between the two cold rolling steps, subjecting the cold rolled sheet to a decarburization and primary recrystallization annealing, subjecting the annealed sheet to a secondary recrystallization annealing is a final annealing step to highly align the secondary recrystallized grains into {110}&lt;001&gt; orientation, and then subjecting the final annealed sheet to a purification annealing to remove the impurities from the steel sheet.
In this case, as the orientation of the secondary recrystallized grain becomes aligned into {110}&lt;001&gt;, the magnetic flux density of the steel sheet becomes higher, but the secondary recrystallized grain is apt to become coarse and consequently the width of magnetic domain in the crystal grain becomes wider to increase the eddy current loss, which tends to degrade the iron loss property. Therefore, there are made various attempts for the purpose of making the secondary recrystallized grains fine. For example, Japanese Patent laid open No. 60-89,521 proposes a method of improving the iron loss property by alternately arranging an acceleration region and a delay region for the recrystallization to increase the occurrence of secondary recrystallized grain and prevent the growth thereof to thereby make the secondary recrystallized grain fine. However, the technique for magnetic domain refinement has recently been established by physical introduction of local strain, whereby the low iron loss is obtained without formation of fine secondary recrystallized grains. As a result, a trend of the technical development is to improve the magnetic flux density.
In this connection, Japanese Patent Application Publication No. 58-50,295 discloses a method of obtaining a high magnetic flux density by giving a onedirectional temperature gradient in the secondary recrystallization to selectively grow secondary recrystallized grains of {110}&lt;001&gt; orientation. In this method, however, the temperature control is very difficult, so that such a method can not be said to be practical.